1. Field of the Invention
The present invention relates to a method of detecting and adjusting exposure conditions of a charged particle exposure system that forms fine patterns on a substrate with electron beams.
2. Description of the Related Art
High integration of integrated circuits in recent years has replaced photolithography that has been a main method of forming fine patterns, with a new exposure method employing electron beams.
A conventional electron beam exposure system deflects a variable rectangular beam to scan a sample wafer and draw a pattern on the wafer with the beam. The exposure system draws a pattern by connecting rectangular exposure shots to one another. The number of exposure shots per unit area becomes larger if the size of each shot is small, and if the number is large, throughput may decrease. To achieve practical throughput in exposing very fine patterns, a block exposure method is employed. This method will be explained.
A semiconductor device such as a 64-M DRAM that comprises very fine patterns is usually made up of repetition of several basic patterns. If each of the basic patterns is formed by one irradiation exposure regardless of its complexity, very fine patterns may be formed at acceptable throughput. To realize this, the block exposure method employs a transmission mask (a stencil mask) on which the basic patterns are formed. By applying an electron beam to the transmission mask, each basic pattern is formed on a sample in one irradiation exposure. This operation is repeated so that connected basic patterns may be formed on the sample.
An example of this method is reported in "IEEE TRANS. ON ELECTRON DEVICES," Vol. ED-26 (1979) 663, and FIG. 3 shows a basic arrangement of the example.
In FIG. 3, an electron gun 3a emits an electron beam, which passes through an irradiation optical system 1 comprising a first rectangular aperture 2 and a lens 3. A pattern selecting deflector 4 positioned at a crossover image point deflects the electron beam, which then irradiates an optional pattern 5 formed on a stencil mask 6. The pattern 5 of the stencil mask 6 forms the electron beam into a patterned electron beam. A lens 9 converges the patterned electron beam to an optical axis. A reduction lens 14 reduces the cross section of the beam. The beam passes through a projection lens 24 and deflecting systems 23, and exposes a wafer 12.
According to this method, the electron beam deflected from the optical axis is returned to the optical axis only by a converging action of the lens 9, and the electron beam passes through a different part of the lens 9 depending on a selected pattern of the stencil mask 6. To arrange as many patterns as possible on the stencil mask 6, an electron beam is required to pass through the lens 9 as far away as possible from the optical axis thereof. Lens aberration existing in this arrangement, however, seriously influences a transferred image. To cope with this problem, the inventors have once proposed an electron beam exposure system of FIG. 4.
In FIG. 4, an irradiation optical system 1 comprises a first rectangular aperture 2, a first lens 3, a deflector 4, and a second lens 58a. A mask deflector 56 is disposed on the incident side to deflect an electron beam toward a required pattern 7 among a group of patterns formed on a stencil mask 6. A mask deflector 57 is disposed on the irradiation side to return the electron beam passed through the stencil mask 6 to an original optical axis. The mask deflectors 56 and 57 form a deflector 55. A reduction optical system 10 comprises a lens 58b, a reduction lens 14, a projection lens 24, a deflector 23, etc. A movable stage 11 carries a substrate such as a wafer which is irradiated with the electron beam. A mask deflector driving unit 59 drives the deflector 55 according to processed data.
The aperture pattern selecting deflectors 56a, 56b, 57a, and 57b deflect an electron beam toward a pattern formed on the stencil mask 6, and return the electron beam to the optical axis under the stencil mask 6. With this arrangement, an electron beam for selecting a mask pattern passes through the centers of the lenses 58a and 58b.
FIG. 5 shows an example of the stencil mask. As shown in a side view of the figure, a pattern forming part of the stencil mask is thinned. Each pattern is formed by etching. A substrate of the stencil mask is made of semiconductor material such as silicon or a metal plate. On the stencil mask, there are formed a plurality of groups of patterns that are selected by the aperture pattern selecting deflector 55. The pattern groups may be moved relative to an optical axis by an X-Y stage that supports the stencil mask. To load the stencil mask onto the X-Y stage, there is arranged a mask loading subchamber that can be disconnected from a column proper by a gate valve.
In this sort of block pattern transfer electron beam exposure system, the reduction optical system 10 including the reduction lens 14, projection lens 24, etc., determines a reduction ratio (magnitude) in reducing and transferring a pattern on the stencil mask onto the wafer, according to a combination of conditions such as the power of the respective lenses, etc. Reducing an image by the lenses usually causes rotation of the image. This rotation can be adjusted by adjusting conditions of the lenses.
In forming a required combination of patterns on a wafer by continuously irradiating a plurality of the same or different patterns formed on a stencil mask, the respective patterns must be connected to one another correctly and continuously on the wafer. Namely, the image of each pattern formed on the wafer must have the same reduction ratio and rotational angle. To improve an accuracy of the size of each image on the wafer and an accuracy of connecting the images to each other, the reduction ratio and rotational angle must be accurately detected.
To detect the reduction ratio and rotational angle, a rectangular electron beam is applied to the wafer, and a sample stage coordinate measuring system measures the size and orientation of the rectangular beam. The size of the rectangular beam formed on the wafer is usually 3 to 4 .mu.m.sup.2 or smaller. Beam edge roughness influences measuring accuracy of the size and orientation of the beam. In this way, the conventional measuring system has a limit in its accuracy.